Water pollution is the addition of any substance to a body of water that might degrade its quality. The list of such substances, or pollutants, is depressingly long: Industrial chemicals like polychlorinated biphenyls (PCBs) and raw sewage get dumped directly into a body of water or wash into it from the land. Meanwhile, contaminants like mercury and acid-forming precursors, along with other air pollutants from fossil fuel combustion or industry, fall back to Earth with the rain and flow as stormwater runoff into rivers, streams, lakes, and seas. Nutrients (from fertilizers and animal waste) and pesticides also enter from farm and lawn runoff. Sediments from soil erosion can flow into surface waters from farms, construction sites, or heavily eroded stream banks that are no longer shored up by a well-rooted plant community. Water pollution is not limited to the introduction of chemicals or sediment; municipal trash often finds its way to rivers, streams, and oceans (see Chapter 7). The heated water released into surface waters near power plants causes thermal pollution, raising the temperature of the water enough to impact many of the organisms that live in that stretch of river, lake, or ocean. Even groundwater sources can become polluted from underground chemical storage tanks or from the movement of surface pollutants down through the soil.

There are two classes of pollution, defined by how they are delivered to the water. Point source pollution is water pollution whose discharge source can be clearly identified (that is, one can “point” to the “source”). This includes pollution from large discharge pipes of wastewater treatment plants or industrial sites. The discharge itself is known as effluent.

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Some of this effluent has the capacity to be quite dangerous. In most cities, sewer pipes run alongside streams because engineers assume that pipe sewage, if placed alongside the stream, will flow in the same direction and therefore reach treatment plants via gravity. But in many urban areas, those pipes are old and beginning to leak. With such a leak, raw sewage flows directly into the stream. The problem is particularly bad in cities of the developing world, where waterborne pathogens (disease-causing organisms) in raw or partially treated human and animal waste represent a leading cause of sickness and death.

But the good news is that because point sources can be easily identified, they can also—at least hypothetically—be remedied.

As researchers would soon discover, the low oxygen levels in the Gulf of Mexico were due to a different and an even more challenging problem: nonpoint source pollution. The origin of nonpoint source pollutants are not easily identifiable. Though some arrive by air, most enter the water from overland flow (stormwater runoff); this means they can come from any part of the land that drains into a given body of water. INFOGRAPHIC 15.1

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Nonpoint source pollutants can include everything from humanmade toxicants to natural substances such as silt, sand, and clay, which can enter the water as sediment pollution (eroded soil that is washed into the water through runoff). To be sure, these substances deliver valuable nutrients to aquatic ecosystems. But excessive amounts of them can cloud the water, making it hard for sunlight to penetrate and thus disrupting photosynthesis. Sediment pollution can also harm organisms directly by clogging gills. And, when it covers the sea or river bottom, sediment can smother the nooks and crannies that serve as habitat or spawning areas.

Something in the Gulf waters—some pollutant—was causing the levels of dissolved oxygen (DO) to plummet—a condition known as hypoxia. Turner and his colleague Nancy Rabalais (the two would later marry) knew that hypoxia was a serious problem. Water can hold only a limited amount of oxygen, much less than found in air. So even a small decrease can have immediate effects on aquatic life. Even underwater organisms need oxygen to survive. Like terrestrial beings, they use it in the process of cellular respiration.

Month after month, year after year, in bigger and better-equipped boats, Turner and Rabalais surveyed the water, mapping out a hypoxic zone that grew from 40 km² (15 square miles) in 1988 to more than 15,000 km² (5,800 square miles) in 2013 (that’s about the size of Connecticut). As they quickly learned, the oil rigs were not the main problem. An excess of the nutrients nitrogen and phosphorus were triggering a process known as eutrophication (or, more precisely, cultural eutrophication, since human activities were the source of these nutrients).

“Imagine stretching a giant sheet of plastic wrap from the Mississippi River’s mouth, straight across to Galveston [Texas],” Turner says. “Now imagine sucking all the air out and leaving the whole ecosystem there to suffocate.”

Here’s how eutrophication works: Because nitrogen and phosphorus fuel plant growth, extra amounts trigger explosions of algae. Though algae produce oxygen through photosynthesis, they also block sunlight from reaching underwater plants, ultimately blocking much more photosynthesis than they conduct. This imbalance causes the plants at the bottom of the water to die en masse. When that happens, the turbidity (cloudiness) of the water increases: Dead and dying plant roots can no longer secure the river or seabed in shallow areas, and bottom sediments can easily enter the water column if disturbed by waves or fast-moving water. This disturbance reduces photosynthesis even more.

From there, it gets even worse. As the plants die, they are consumed by bacterial decomposers, triggering yet another bloom—a bacterial one. The bacterial populations increase rapidly, consume oxygen as they digest the dead plants, and quickly deplete any remaining oxygen in the water. INFOGRAPHIC 15.2

“Imagine stretching a giant sheet of plastic wrap from the Mississippi River’s mouth, straight across to Galveston [Texas]. Now imagine sucking all the air out and leaving the whole ecosystem there to suffocate”—Eugene Turner.

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But where was all that extra nitrogen and phosphorus coming from in the first place?